In order for automobile exhaust catalysts to function they must be at operational temperatures. When an engine starts from cold the vehicle, including the exhaust system and any after-treatment device located therein, is at ambient temperature. Since 60% to 80% of the total emissions emitted occur at engine cold start and idle up to 120 seconds, it is imperative that the catalyst function as fast as possible. This invention provides a passive thermatically controlled bypass valve to enable the exhaust flow to bypass the enthalpy loss incurred by driving the turbine wheel of a turbocharger, and thus more rapidly deliver heat to the catalyst or after-treatment.